The invention relates to the power drive of heavy vehicles, particularly to the hydraulic-mechanical transmission of power from the vehicle engine to the shafts to the traction wheels. It relates more especially to the use of a hydraulic pump coupled to the vehicle engine adapted to supply liquid under pressure to two hydraulic motors adapted to transmit torque to the traction wheels or to the tracks on both sides of the vehicle via a planetary gear unit.
The known hydraulic power transmissions in heavy vehicles comprise a hydraulic pump driven by the engine and supplying liquid under pressure to a hydraulic motor which is coupled to the traction wheels, in many instances through a variable gear transmission. The system is especially useful in the case of tracked vehicles provided with hydraulic motors on both sides, whereby steering of the tracked vehicle is performed by having the two motors rotating at different speeds, whenever a bend in the road is to be navigated. Both hydraulic pump and motors are of the variable displacement type, permitting varying of the output torque and the revolutions in accordance with the road resistance and the vehicle speed.
This system replaces to a large extent the well-known automatic gear transmissions which are used in cars and light trucks, which would, however, be of excessive size in the case of heavy vehicles, especially tracked vehicles driven by engines having an output of several hundreds of horsepowers.
The size of the pump and the motor is determined by the maximum torque delivered by the engine and the maximum revolutions required to drive the vehicle. An IC-engine characteristic is in the shape of a curve "a" as illustrated in FIG. 1, which shows a high torque at low speed and a low torque at high speed. A hydraulic, variable-displacment motor adapted to transmit the torque at the respective revolutions has very large dimensions, since its maximum output required (at point "b" of FIG. 1) equals the maximum torque times maximum velocity, which is much higher than the maximum engine output at any speed, and the hydraulic motor cannot be used efficiently and its power is wasted.
Of course, by placing a mechanical gear transmission between the hydraulic motor and the output shaft, it is possible to increase the vehicle speed without having to increase the motor displacement, but this system distracts from the advantages of the stepless speed possible with the hydraulic system and be not advance compared with the conventional automatic gear transmission. FIG. 2 shows a diagram of the output of a hydraulic motor in conjunction with a three-step transmission.
It is the object of the present invention to replace one large hydraulic motor by two hydraulic motors of smaller output each.
It is another object to provide three different gear ratios at different outputs and revolutions of the two hydraulic motors.
And it is a final object to provide a hydraulic-mechanical power transmission of simple design at relatively low cost, and to provide simple, automatic control means adapted to utilize the engine power at its most efficient output and speed.